Reticles are often used to form patterns on integrated circuit wafers. As pattern dimensions decrease, critical dimension (CD) proximity effect becomes a significant problem and methods to correct for the proximity effect problems are used. Optical proximity correction (OPC) is a method of eliminating deviations in the critical dimensions of a device due to a proximity effect. The proximity effect occurs when radiation, such as light, is projected onto a reticle having a pattern thereon. Due to diffraction of the radiation by the reticle, which may also be characterized as scattering, the radiation diverges and spreads. The diffracted light creates multiple diffraction orders, not all of which are captured by a lens of an optics system. The lens captures a portion of the light, which is directed to a surface of a semiconductor substrate. In addition, a portion of the radiation passing through a photoresist material on the semiconductor substrate is reflected by the surface of the underlying semiconductor substrate, causing light interference and leading to overexposure of the photoresist material in part of the pattern, which causes defects, such as optical distortions (i.e., rounding), in the photoresist material. While OPC methods are used to correct for these defects, conventional OPC methods are complicated because computer software must be utilized to calculate where the optical distortions are likely to occur. Conventional OPC methods also rely on empirical data. However, empirical-based solutions to OPC require protracted time, and many mask iterations, in order to be successfully developed.
Assist features, such as serif features, hammerhead features, and outrigger features, are also used in conventional OPC methods. The assist features are formed at sub-resolution scale relative to the patterns on the reticle, which correspond to the features to be formed on the semiconductor substrate. While conventional assist features are not imaged (e.g., printed) on the semiconductor substrate, these assist features cause additional diffraction and scattering of the radiation due to the production of diffractive signals that may be at a high angle of attack and go through edges of a lens in the optics system. The conventional assist features are sensitive to aberrations since they enable imaging of the features on the semiconductor substrate but do not, themselves, participate in the imaging. Use of such assist features in conventional OPC methods causes problems in CD uniformity (CDU) and CD bias control in both production and simulation/modeling.
It would be desirable to achieve a photolithography process having fewer mask manufacturing problems, imaging quality problems, CD, and CDU problems.